The main reason why fish can’t live out of water is because they are unable to breathe! While your body needs to have air in order to breathe, a fish’s body needs to have water. The air you take in through your nose or mouth goes into your lungs, which help your body get what it needs from the air you’ve breathed in -- the part of the air called “oxygen.” Just like your body, the body of a fish also needs oxygen. The only difference is that fish get the oxygen from water because instead of having lungs as helpers fish have something called gills. The fish’s gills can only take oxygen from water and not air, just like your lungs can only take oxygen from air and not water!
So, just like you open your mouth to take in air when you breathe, a fish opens its mouth to take in water! And that’s why a fish cannot live out of water: it’s no longer able to breathe because there isn’t any water for it to take in.